1. Field of the Invention
The present invention relates to a running control apparatus, which controls a vehicle to a target running state, and a running control program executed in a computer to operate this apparatus.
2. Description of Related Art
A running control apparatus is disposed in a vehicle to control an actual speed of the vehicle to a target speed. As the running control of this apparatus, cruise control and adaptive cruise control are, for example, well known. In cruise control, the running speed of the vehicle is maintained to a value set by a driver. In adaptive cruise control, the running speed of the vehicle is automatically controlled to an appropriate value in accordance with information about distance to another vehicle.
In this apparatus, when the driver operates the braking device during the running control, this running control is generally stopped, and the vehicle is run under control of the braking device. Further, the apparatus can be structured such that the running control is restarted when the operation of the braking device is ended.
For example, Published Japanese Patent First Publication No. 2005-255146 discloses a vehicle deceleration control apparatus wherein deceleration control is performed for a present vehicle in accordance with the distance to another vehicle located in front of the present vehicle. In this apparatus, no deceleration control is performed just after the operation of the braking device is finished.
The reason that no deceleration control is performed will be described. When the vehicle is under specific running circumstances just after the end of the braking operation, the deceleration control restarted after the end of the braking operation sometimes causes discomfort to the driver. More specifically, the running vehicle receives a running resistance including a hill climbing resistance in accordance with running circumstances of the vehicle. Therefore, to run the vehicle at a constant speed, it is required to drive the vehicle at a driving force against the running resistance. When deceleration control (representing the running control) stopped during the running of the vehicle on a non-inclined road is, for example, restarted during the running of the vehicle on an upward slope, the running resistance acting on the vehicle at the restart time becomes higher than that at the stop time. In this case, the vehicle is excessively decelerated, so that the running speed of the vehicle undesirably becomes lower than a target value. When acceleration control is performed to quickly heighten the running speed to the target value, the running speed of the vehicle easily exceeds the target value. Therefore, the alternation of the deceleration control and the acceleration control is repeatedly performed to cause hunting in the vehicle engine and to generate vibration of the vehicle body. In contrast, when the deceleration control is restarted during the running of the vehicle on a downward slope, deceleration of the vehicle becomes insufficient. In this case, it takes a long time to lower the running speed to the target value.